It is common practice to install transmission lines underground through a pipeline. This is often accomplished by progressively joining 30 to 50 ft. lengths of pipe end to end; lowering the assembled pipeline into a trench; and then threading the transmission line through the pipeline. However, manholes are required at 2,000 to 4,000 ft. intervals along the pipeline to permit splicing and threading of the insulated conductors. This method is obviously limited to applications where the pipeline is accessable at regular and comparatively short intervals. It cannot be used in many applications where manholes cannot be installed. Thus, the method is also impractical for emplacing transmission lines in many underwater installations.
One method that is used for submarine installations consists of sequentially joining sections of pipe and electrical conductors on land and then pulling the entire pipeline into position. However, this method must be modified when the transmission line is of any great length. As an example, a 3-mile crossing to an off-shore nuclear power plant may require several hundred lengths of pipe. This length of pipe cannot be pulled from shore in one operation because of the tremendous drag forces, not to mention probable space limitations. Therefore, the pipeline may be made up in five or more sections which are laid out side by side, each containing a transmission cable. The first section is pulled toward the off-shore power plant until its trailing edge is at the shoreline. The transmission cable and pipeline of a second section is then attached to the first and the procedure is repeated until all sections are joined to span the crossing.
The foregoing method and installation presents several disadvantages. First, repairs cannot be made in the event of either external damage to the pipeline or internal electrical breakdown; and, secondly, external damage to the pipeline may result in an "oil spill" since power transmission lines are usually filled with oil as an insulation and coolant.